1. Field of the Invention
This invention relates to light emitting diodes and more particularly to device structures for providing enhanced light extraction efficiency, improved thermal management characteristics, and improved service life.
2. Background Information
Light emitting diodes (LEDs) are a well known class of solid state devices for converting electric energy to light and commonly comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light, which travels in all directions until it either escapes at an exposed surface, or is reabsorbed.
In U.S. Pat. No. 5,779,924 Krames et al. teach the use of surface texturing on an LED to increase light extraction efficiency through a transparent window layer on a top surface of the LED. This surface texturing has a periodicity and depth on the order of a wavelength of the emitted light. Krames et al. do not teach the use of deep pits extending from a transparent window layer down to a depth beneath an optically active layer of the device.
In U.S. Pat. No. 6,410,942, Thibeault et al. teach that an array of individual LEDs on a single chip may provide enhanced light extraction if the distance light travels inside the semiconductor material before reaching a free surface is reduced. This minimizes internal reabsorption of light. Although Thibeault et al. teach the use of a semitransparent conductive contact layer, their teaching does not include a protective passivating layer over their contact layer, which is thus exposed to environmental degradation. Moreover, Thibeault et al specifically teach structures in which a high index of refraction light extracting element is disposed across an emitting junction in an LED structure.
In U.S. Pat. No. 6,518,598, Chen teaches that a spiral pattern can be cut through the active region of a single LED to enhance light extraction. Topologically, Chen's structure is equivalent to a single elongated mesa configured for light escape primarily from the sidewalls of the mesa. Chen does not disclose a plurality of penetrations that extend through the active region of a single LED mesa. Moreover, Chen's spiral designs define a light path, in which the light experiences multiple total internal reflections, extending along the entire length of the spiral. Light trapped in this mode will experience a pathway that is many times the longitudinal dimensions of the chip.